The present invention relates to a process and an apparatus for laminating pre-forms into laminated glazing assemblies, wherein the pre-forms include two solid panes laid on top of each other with a layer of adhesive in between such that the two solid panes with the layer of adhesive in between are subjected to pressure and temperature differences to form the laminated glazing assembly.
The expression xe2x80x9claminated glazing assembliesxe2x80x9d should be understood here to mean multi-layer products which consist of at least two solid panes with an adhesive film in between. The adhesive film is made of plastic or similar material for joining the two solid panes together. The two solid panes may both be made of glass or may both be made of plastic. However, it is also possible for a glass pane and a plastic pane to be joined together by an adhesive film. The individual layers (i.e., the solid panes and the layers or films of plastic) are produced separately in a conventional manner. The individual layers are then layered on top of each other to form of a packet of layers (i.e., a xe2x80x9cpre-formxe2x80x9d), before a final assembly into a laminated glazing assembly.
Modern glazing assemblies made of laminated glass, such as windscreens and side windows in motor vehicles, are often finally laminated in large autoclaves at high pressure and high temperature. Because of the necessary increase in pressure and of the long residence time required, a process using an autoclave step slows down the continuous and highly mechanized sequence of operations in the manufacture of the laminated glass assemblies, which has already been achieved for the manufacture and conversion of the individual layers for forming the pre-forms.
However, processes have also already been described for the manufacture of laminated glass which dispense with the autoclaving for the final bonding. In these autoclave-less processes, the pre-forms are finally bonded at atmospheric pressure.
German Patent No. 2,209,642 (hereinafter xe2x80x9cthe DE ""642 patentxe2x80x9d) describes a process for manufacturing three-layer laminated-glass glazing assemblies. During the process of the DE ""642 patent, the pre-forms are put under vacuum and heated in a chamber, while the intermediate spaces are simultaneously subjected to vacuum using a suction frame. After having reached a certain temperature, for example, 100xc2x0 C., the chamber is again vented to atmospheric pressure. However, the lateral vacuum is maintained by the suction frame, which continues to create a vacuum. Consequently, a primary laminate is simply assembled temporarily. This primary laminate is then transferred, with the suction frame under vacuum, into another chamber which is at atmospheric pressure. In the atmospheric pressure chamber, the primary laminate is heated to a temperature above the melting point of the adhesive film, which has been placed between the two solid panes. The pressure difference between the outside and the inside is sufficient to form the final laminated assembly.
German Patent No. 3,044,717 (hereinafter xe2x80x9cthe DE ""717 patentxe2x80x9d) describes another autoclave-less process for the manufacture of two-layer laminated products having a thermoplastic sheet located on the outside. According to the process of the DE ""717 patent, a chamber at ambient pressure receives the pre-form. The ambient pressure chamber has its pressure temporarily decreased to a pressure even lower than the internal pressure obtained between the layers of the pre-form. The pressure may be reduced with the aid of a suction frame. The pressure difference briefly separates the layers from each other so as to facilitate the outgassing of the intermediate space. Next, the entirety of the pre-form is heated to the bonding temperature. During the time the pre-form is heated to the bonding temperature, the pressure in the chamber is increased back up to ambient pressure so as that the final laminated glazing assembly results and the internal pressure within the pre-form remains below atmospheric.
The two above-described autoclave-less processes rely on separable vacuum systems and also require a suction frame for sealing the packet of layers around the edge. The edge sealing is done, not only during the phase when the pre-form is under vacuum, but also during the subsequent phase wherein the final laminated glazing assembly is produced at atmospheric or higher pressure. For each shape and/or size of the pre-form, depending on the case, a specific variant of the suction frame has to be used. Since positioning and removing the suction frame cannot presently be mechanized, the above-described autoclave-less processes cannot be incorporated into, or can only be incorporated insufficiently into, an at least quasi-continuous manufacturing line. The term xe2x80x9cquasi-continuousxe2x80x9d should be understood here to mean a sequence of operations during which the flow of product material takes place, in principle, continuously during certain pane cycle periods and must only be interrupted for a short time for certain conversion steps, for example, for pressing between membranes or other pressing molds.
German Reference No. 2,424,085 (hereinafter xe2x80x9cthe DE ""085 referencexe2x80x9d) also describes an autoclave-less process during which a plastic film is laminated onto a pane using pressing membranes. In order to carry out the process of the DE ""085 reference, the pane and the film are placed in a chamber so that the pane does not contact the film. Next, the chamber is closed and a vacuum is created therein. The plastic film rests on a membrane forming a wall of the chamber. The membrane is subjected to vacuum on both sides (i.e., on a front and rear face away from the chamber). After a necessary vacuum has been obtained in the chamber, the rear face of the membrane is subjected to atmospheric pressure. Because of the pressure difference established, the membrane presses the film against the pane. The film is made of either self-adhesive plastic or is provided with an adhesive layer. In this way, it is possible to produce curved glazing assemblies made of laminated glass. The curved laminated glazing assemblies are then pressed in a similar manner between two membranes.
When a heat-activatable adhesive is used, the pane is pre-heated outside of the chamber and then placed in the chamber. This process has the advantage that both a suction frame and individual pressing molds may be dispensed with.
Finally, German Reference No. 19,712,145 (hereinafter xe2x80x9cthe DE ""145 referencexe2x80x9d) describes another laminating process during which a pre-form, consisting of two panes and an adhesive film interlayer, is heated to the assembly temperature. The pre-form is then transferred into a chamber and a vacuum is created therein. PVB adhesive films are usually employed for the manufacture of glazing assemblies of the DE ""145 reference. The PVB adhesive films are usually made of laminated glass which has a relatively rough and non-transparent surface so as to guarantee that, after the solid panes and the adhesive film have been joined together, all the air can be sucked out through the channels formed by the unevenness in the film. In order to obtain the required high transparency of the final product, the adhesive film must be fully melted so that the unevenness completely disappears. In the process of the DE ""145 reference, after the minimum pressure has been obtained and after sufficient venting, the edge of the pre-form is sealed by mechanical pressing, while the chamber is vented again. Under the action of forces exerted on the main surfaces of the laminated product, preferably in combination with additional heating in order to completely melt the adhesive film, the rise in pressure causes the panes to be joined to the adhesive film so that a virtually continuous flow of material can be achieved.
The objective of the present invention is to provide an apparatus and process for laminating pre-forms into laminated glazing assemblies, wherein at least a quasi-continuous flow of material is achieved for laminated glazing assemblies of variable outlines and sizes.
The above-described objective is achieved according to the present invention by providing a process for laminating pre-forms into laminated glazing assemblies by using the effect of pressure and heat differences on the superposed layers of the pre-forms in order to form the laminated glazing assemblies. The process comprising the steps of: subjecting the pre-forms to an overall vacuum by lowering the ambient pressure; maintaining the ambient vacuum, while both heating the pre-forms to a temperature necessary for joining the glass to the plastic and pressing the pre-forms between flexible pressing pieces in order to form the laminated glazing assemblies; and increasing the ambient pressure to atmospheric pressure by venting the laminated glazing assemblies.
During the step of subjecting the pre-forms to the overall vacuum, the temperature of the pre-forms is not yet increased or in some cases, is increased only slightly. Thus, any premature bonding is avoided so that the laminated glazing assemblies do not have any air inclusions therein and thus, it is not necessary to scrap the defective laminated glazing assemblies so as to reduce the efficiency of the process.
The second and third step of heating the pre-forms to a temperature necessary for joining the glass to the plastic and pressing the pre-forms between flexible pressing pieces in order to form the laminated glazing assemblies (both while maintaining vacuum), makes it possible to dispense with a suction frame for the pre-forms. During the heating step, the pre-forms are heated to a temperature necessary to join the glass to the plastic. An adhesive film of polyvinylbutyral (i.e., xe2x80x9cPVBxe2x80x9d), which is used for most multi-layer laminated glazing assemblies, melts at approximately 130xc2x0 C. When the layer of adhesive is of another plastic other than PVB, the layer of adhesive may be on the outside of the solid panes, may be a heat-activatable adhesive between the solid panes, and is usually of a type having a melting point less than 130xc2x0 C.
The cycle time of the process and of the corresponding apparatus is essentially determined by the time required for the pre-form to be subjected to vacuum during the initial step. This cycle time has been found sufficient to heat the pre-form to the required temperature during the heating step. It also allows enough time for the pressing step, which may require the material between the pressing pieces to be immobilized, at least for a short period of time lasting a few seconds.
The rapid final cooling of the laminated glazing assembly to normal temperatures, which occurs after the exit shutter is opened, is not critical when the solid panes are glass because the assembly temperatures are also significantly below the usual pre-tensioning temperatures.
Preferably, the pre-form is reheated, in any known manner, so as to completely melt the structure of the film to assemble the laminated glazing assembly.
In order to achieve a high efficiency, the various steps, namely the evacuation, heating, pressing and venting, are carried out in successive physically separate chambers, wherein the heating chamber and the pressing chamber always remain under vacuum.
The level of vacuum created in the heating chamber and the pressing chamber is approximately 1 to 10 hPa of absolute pressure. The entry temperature of the pre-form is brought to approximately 50xc2x0 C. by pre-heating the preform. Thereafter, in the apparatus, the heating rate is set to approximately 2xc2x0 C./second until the usual assembly temperature of 130xc2x0 C. is reached. The cycle time of a complete cycle is about 15 seconds, this being imposed by the prior and subsequent processes. The pressing time itself is only about 5 seconds.
Even when taking into account the reheating of the pre-form in order to form the final laminated glazing assembly, the total times for manufacturing laminated glazing assemblies according to the process of the present invention are appreciably shorter than the values of approximately 2.5 hours that are achieved with the autoclaving process.
The flexible pressing pieces used in the pressing step are most likely conventional membranes. Therefore, the plant does not require pressing molds of any one specific type, which are set for certain glazing models. Laminated glazing assemblies of different outlines and sizes can be treated directly, one after another, without having to modify the plant and therefore, without any down time.
Flat pre-forms may be pressed between a membrane and a flat and solid bearing surface, whereas curved glazing pre-forms are preferably pressed in a conventional manner between two membranes that can be pressed on both sides in order to carry out the final assembly. Of course, flat pre-forms may also be pressed by two membranes in a pressing chamber.
In all cases, during the pressing of the pre-forms, the pre-forms may rest on a conveyor belt assembly in the pressing chamber. The conveyor belt, of the conveyor belt assembly, transfers the pressing or reaction forces to the main surface of the pre-form facing the conveyor belt.
Of course, it is also possible to use other pressing pieces such as, for example, resilient rollers. However, pressing membranes transfer the pressing effect over the entire surface of the pre-form in the most homogeneous manner. The pressing membranes also adapt best to the surfaces and sizes of different glazing assemblies. However, care has to be taken to ensure that the pre-forms to be pressed are always entirely covered by the membranes. The size of the pre-forms that can be converted therefore depends directly on the membrane area available. In all cases, the pressing pieces are required to provide a covering, which reliably prevents adhesion of the glass or of the plastic, in a conventional manner.
The pressing membranes may be subjected to atmospheric pressure on the opposite side from the pre-form because there is a vacuum inside the pressing chamber. In all cases, this pressure difference is enough to generate sufficiently high assembly forces. It is therefore possible, in principle, to avoid the need for and the use of a high-pressure generator.
During the venting of the laminate, in order to prevent air from getting back into the laminated product, it is necessary, in principle, to find a way to bond the edges in a manner similar to the process mentioned in the DE ""145 reference. When membranes are used that have working areas which extend beyond the edges of the pre-forms on all sides thereof, a greater force is exerted on the edges of the pre-forms during pressing to result in sufficient bonding of the edges. This is especially so because the adhesive film reaches its melting point along the edges rather than in the middle of the surface. The vacuum created is as if it were trapped in the laminate.